The present invention relates generally to acoustic levitation and concentration and, more particularly, to the use of a hollow, cylindrical piezoelectric crystal for levitation and concentration which requires only low power and obviates the need for exact alignment of parts to generate standing waves.
Acoustic levitation provides a means for isolating small samples of particles having diameters less than several millimeters without the influence of a containment vessel (See, e.g., E. H. Trinh, xe2x80x9cCompact Acoustic Levitation Device For Studies In Fluid Dynamics And Material Science In The Laboratory And Microgravityxe2x80x9d Rev. Sci. Instrum. 56, 2059-2065 (1985), D. B. Thiessen and P. L. Marston, xe2x80x9cPrinciples Of Some Acoustical, Electrical, And Optical Manipulation Methods With Applications To Drops, Bubbles, And Capillary Bridgesxe2x80x9d ASME Fluids Eng. Div. Publ. FED (1998), M. A. H. Weiser and R. E. Apfel, xe2x80x9cExtension Of Acoustic Levitation To Include The Study Of Micron-Size Particles In A More Compressible Host Liquidxe2x80x9d J. Acoust. Soc. Am. 71,1261-1268 (1982), E. G. Lierke et al., xe2x80x9cAcoustic Positioning For Space Processing Of Materials Science Samples In Mirror Furnacesxe2x80x9d in IEEE Ultrasonics Symposium 1129-1139 (1983), K. Yasuda, xe2x80x9cBlood Concentration By Superposition Of Higher Harmonics Of Ultrasoundxe2x80x9d Jpn. J. Appl. Phys. 36, 3130-3135 (1997), Ph. Caperan et al., xe2x80x9cAcoustic Agglomeration Of A Glycol For Aerosol: Influence Of Particle Concentration And Intensity Of The Sound Field At Two Frequenciesxe2x80x9d J. Aerosol Sci. 26, 595-612 (1995), G. Whitworth et al., xe2x80x9cTransport And Harvesting Of Suspended Particles Using Modulated Ultrasoundxe2x80x9d Ultrasonics 29, 439-444 (1991), and K. M. Martin and O. A. Ezekoye, xe2x80x9cAcoustic Filtration And Sedimentation Of Soot Particlesxe2x80x9d Experiments in Fluids 23, 483488 (1997)). Most acoustic levitation devices operate by localizing a sample near the nodal planes of an acoustic standing wave. This has proven to be a viable technique for measuring material properties of small sample quantities (e.g. droplets, aerosols, etc.) without obscuring the results with the effects of, a mounting apparatus (See, e.g., M. A. H. Weiser and R. E. Apfel, supra, and E. G. Lierke et al., supra). Other applications include the use of acoustic forces to concentrate aerosols and/or particulates near the nodal planes of the field for harvesting or sedimentation purposes. Advances in the design of acoustic levitators over the past several decades have proven useful for applications where samples may reside in either gaseous or liquid host media.
The standing-wave field produced by an acoustic levitation device is strongly dependent upon the spatial alignment of the system components and often requires moderate to high electrical input power levels to drive the acoustic generators and achieve the desired levitation. This is especially true for levitating solid and liquid samples in air. The large acoustic impedance mismatch between the displacement-generating device and the air medium is often a difficult problem to overcome. Resonant transduction devices having Q  greater than 1000 have been built to address this problem and have proven quite useful when electrical power efficiency is not a limiting factor (See, e.g., D. B. Thiessen and P. L. Marston, supra, and J. A. Gailego Juarez and G. Rodriguez Corral xe2x80x9cPiezoelectric Transducer For Air-Borne Ultrasoundxe2x80x9d Acustica 29, 234-239 (1973)).
Piezoelectric cylinders have received significant attention by industry. Such crystals are used for vibration damping, sources for sonar, sensors and actuators, motors and X-Y micropositioners, to name several uses.
Accordingly, it is an object of the present invention to provide an apparatus for efficiently achieving acoustic levitation and concentration which in its simplest embodiment is free from the requirement of careful alignment of its component members.
Additional objects, advantages and novel features of the invention will be set forth, in part, in the description that follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects of the present invention, and in accordance with its purposes, as embodied and broadly described herein, the method for concentrating particles suspended in a fluid hereof includes the steps of: matching the breathing-mode acoustic resonance of a hollow cylindrical piezoelectric transducer to the acoustic resonance of the interior volume thereof when filled with the fluid; applying periodic electrical excitation thereto such that resonant acoustic waves are generated in the interior volume of the cylindrical piezoelectric transducer; and subjecting the fluid having particles suspended therein to the equilibrium force pattern formed by the resonant acoustic waves such that the particles move to the region of the equilibrium force pattern and are concentrated thereby.
It is preferred that the step of matching the breathing-mode acoustic resonance of the cylindrical piezoelectric transducer to the acoustic resonance of the interior thereof when filled with the fluid is achieved by inserting a cylindrical-shaped shaped rod into the piezoelectric cylinder such that the axis of the cylindrical-shaped shaped rod is collinear with the axis of the piezoelectric cylinder.
Preferably, the length of the cylindrical-shaped rod is approximately the length of the cylindrical piezoelectric transducer and the diameter of the rod is chosen such that the length of the annular space between the cylindrical rod and the hollow cylindrical piezoelectric transducer along a radius thereof is an integral number of half-wavelengths of sound in the fluid inside of the annular space at the resonant frequency of the piezoelectric transducer.
In another aspect of the present invention in accordance with its objects and purposes the method for levitating particles in a fluid hereof includes the steps of: matching the breathing-mode acoustic resonance of a hollow, cylindrical piezoelectric transducer to the acoustic resonance of the interior thereof when filled with the fluid; applying periodic electrical excitation to the surface of the cylindrical piezoelectric transducer such that resonant acoustic waves are generated in the interior of the cylindrical piezoelectric transducer; and subjecting the particles to the equilibrium force pattern formed by the resonant acoustic waves such that the particles are levitated by the equilibrium force pattern.
Preferably the step of matching the breathing-mode acoustic resonance of a cylindrical piezoelectric transducer to the acoustic resonance of the interior thereof when filled with the fluid is achieved by inserting a cylindrical-shaped rod into the piezoelectric cylinder such that the axis of the cylindrical-shaped rod is collinear with the axis of the piezoelectric cylinder.
It is preferred that the length of the cylindrical-shaped rod is approximately the length of the cylindrical piezoelectric transducer and that the diameter of the rod is chosen such that the length of the annular space between the cylindrical insert and the hollow cylindrical piezoelectric transducer along a radius thereof is an integral number of half-wavelengths of sound in the fluid inside of the annular space at the resonant frequency of the cylindrical piezoelectric transducer.
Preferably also the fluid includes air.
In yet a further aspect of the present invention, in accordance with its objects and purposes the apparatus for concentrating particles suspended or entrained in a fluid hereof includes in combination: a cylindrical piezoelectric transducer having a hollow interior portion and wherein the breathing-mode acoustic resonance of the cylindrical piezoelectric transducer is matched to the acoustic resonance of the interior portion thereof when the interior portion is filled with the fluid; a function generator for applying periodic electrical excitation to the surface of the cylindrical piezoelectric transducer such that resonant acoustic waves in are generated in the hollow interior portion of the cylindrical piezoelectric transducer; and means for introducing the fluid having particles suspended or entrained therein into the region of the equilibrium force pattern formed by the resonant acoustic waves such that the particles move to the region of the equilibrium force pattern and are concentrated thereby.
Preferably, the apparatus includes a cylindrical-shaped rod disposed in the interior portion of said piezoelectric cylinder such that the axis of the cylindrical-shaped rod is collinear with the axis of the piezoelectric cylinder and forming thereby an annular region, whereby the breathing-mode acoustic resonance of the cylindrical piezoelectric transducer is matched to the acoustic resonance of the interior portion thereof when filled with said fluid.
It is preferred that the length of said cylindrical-shaped rod is approximately the length of the cylindrical piezoelectric transducer and that the diameter of the rod is chosen such that the length of the annular space between the cylindrical rod and the hollow cylindrical piezoelectric transducer along a radius thereof is an integral number of half-wavelengths of sound in the fluid inside of the annular space at the resonant frequency of the cylindrical piezoelectric transducer.
It is also preferred that the diameter of the cylindrical-shaped rod is twice the wavelength of sound within the fluid inside of the annular space at the resonant frequency of the piezoelectric transducer, whereby a single equilibrium force pattern is generated within the annular region.
In still another aspect of the invention in accordance with its objects and purposes the apparatus for levitating particles in a fluid hereof includes: a cylindrical piezoelectric transducer having a hollow interior portion such that the breathing-mode mode acoustic resonance thereof is matched to the acoustic resonance of the hollow interior portion thereof when filled with the fluid; a function generator for applying periodic electrical excitation to the surface of the cylindrical piezoelectric transducer whereby resonant acoustic waves are generated in the hollow interior portion of the cylindrical piezoelectric transducer; and means for introducing the particles into the equilibrium force pattern formed by the resonant acoustic waves such that the particles are suspended by the equilibrium force pattern.
Preferably, the apparatus includes a cylindrical-shaped rod disposed in the hollow interior portion of the piezoelectric cylinder such that the axis of the cylindrical-shaped rod is collinear with the axis of the piezoelectric cylinder forming thereby an annular region, whereby the breathing-mode acoustic resonance of the cylindrical piezoelectric transducer is matched with the acoustic resonance of the interior thereof when filled with the fluid.
It is preferred that the length of the cylindrical-shaped rod is approximately the length of the cylindrical piezoelectric transducer and that the diameter of the cylindrical-shaped rod is chosen such that the length of the annular region between the cylindrical insert and the hollow cylindrical piezoelectric transducer along a radius thereof is an integral number of half-wavelengths of sound in the fluid inside of the annular region at the resonant frequency of the cylindrical piezoelectric transducer.
Benefits and advantages of the present invention include low power operation, the commercial availability of hollow piezoelectric cylinders in numerous materials and sizes and freedom from exacting alignment requirements for component parts.